A. Glass Substrates for Flat Panel Displays
Manufacturers of flat panel displays, such as, liquid crystal displays (LCDs), use glass substrates to produce multiple displays simultaneously, e.g., six or more displays at one time. The width of a substrate limits the number of displays that can be produced on a single substrate, and thus wider substrates correspond to increased economies of scale. Also, display manufacturers need wider substrates to satisfy a growing demand for larger size displays.
In addition, such manufacturers are seeking glass substrates that can be used with polycrystalline silicon devices that are processed at higher temperatures (hereinafter referred to as “poly-silicon” applications). In particular, a need exists for high strain point glass compositions that do not undergo compaction during display manufacture. Such glasses generally require higher forming temperatures, thus leading to a need for improved forming structures (isopipes) for use in the fusion process.
B. Fusion Process
The fusion process is one of the basic techniques used in the glass making art to produce sheet glass. See, for example, Varshneya, Arun K., “Flat Glass,” Fundamentals of Inorganic Glasses, Academic Press, Inc., Boston, 1994, Chapter 20, Section 4.2, 534-540. Compared to other processes known in the art, e.g., the float and slot draw processes, the fusion process produces glass sheets whose surfaces have superior flatness and smoothness. As a result, the fusion process has become of particular importance in the production of the glass substrates used in the manufacture of flat panel display devices, e.g., liquid crystal displays (LCDs).
The fusion process, specifically, the overflow downdraw fusion process, is the subject of commonly-assigned U.S. Pat. Nos. 3,338,696 and 3,682,609, to Stuart M. Dockerty, the contents of which are incorporated herein by reference. A schematic drawing of the process of these patents is shown in FIG. 1. As illustrated therein, the system includes a supply pipe 9 which provides molten glass to a collection trough 11 formed in a free-space spanning, refractory body 13 known as an “isopipe.”
Once steady state operation has been achieved, molten glass passes from the supply pipe to the trough and then overflows the weirs 19 (i.e., the tops of the trough on both sides; see FIGS. 2 and 3), thus forming two sheets of glass that flow downward and inward along the outer surfaces of the isopipe. The two sheets meet at the bottom or root 15 of the isopipe, where they fuse together into a single sheet, e.g., a sheet having a thickness of ˜700 microns. The single sheet is then fed to drawing equipment (represented schematically by arrows 17 in FIG. 1), which controls the thickness of the sheet by the rate at which the sheet is drawn away from the root.
A vertical temperature gradient imposed on the isopipe is used to manage the viscosity of the glass. At the root of the isopipe, the glass viscosity is typically in the range of approximately 100 to 300 kP.
As can be seen in FIG. 1, the outer surfaces of the final glass sheet do not contact any part of the outside surface of the isopipe during any part of the process. Rather, these surfaces only see the ambient atmosphere. The inner surfaces of the two half sheets which form the final sheet do contact the isopipe, but those inner surfaces fuse together at the root of the isopipe and are thus buried in the body of the final sheet. In this way, the superior properties of the outer surfaces of the final sheet are achieved.
As is evident from the foregoing, isopipe 13 is critical to the success of the fusion process as it makes direct contact with the glass during the forming process. Thus, the isopipe needs to fulfill strict chemical and mechanical requirements to have a lifetime that is not too short and to deliver a quality sheet glass product. For example, the isopipe should not be rapidly attacked by or be the source of defects in the glass. Also, it should be able to withstand a vertical temperature gradient of, for example, 100° C. during use, and transient gradients larger than that during heat up. In addition, the rate of deflection due to creep at the use temperature should be low.
In particular, the dimensional stability of the isopipe is of great importance since changes in isopipe geometry affect the overall success of the fusion process. See, for example, Overman, U.S. Pat. No. 3,437,470, and Japanese Patent Publication No. 11-246230.
Significantly, the conditions under which the isopipe is used make it susceptible to dimensional changes. Thus, the isopipe operates at elevated temperatures on the order of 1000° C. and above. Moreover, the isopipe operates at these elevated temperatures while supporting its own weight as well as the weight of the molten glass overflowing its sides and in trough 11, and at least some tensional force that is transferred back to the isopipe through the fused glass as it is being drawn. Depending on the width of the glass sheets that are to be produced, the isopipe can have an unsupported length of two meters or more. Current business trends are towards ever larger glass sheets requiring ever larger isopipes for their formation. For an isopipe span on the order of 13 feet, the weight of an isopipe made from zircon (see below) is estimated to be in excess of 15,000 pounds.
C. Zircon Isopipes
To withstand the above demanding conditions, isopipes 13 have been manufactured from isostatically pressed blocks of refractory material (hence the name “iso-pipe”). In particular, isostatically-pressed zircon refractories, such as those sold by St. Gobain-SEFPRO of Louisville, Ky., have been used to form isopipes for the fusion process.
Use of a zircon isopipe limits the fusion process in two ways. First, zircon dissolves into the glass at hotter regions near the weirs of the isopipe, and then precipitates in the cooler regions near the root to form secondary zircon crystals. See U.S. Patent Publication No. 2003/0121287, published Jul. 3, 2003, the contents of which are incorporated herein by reference. These crystals can be sheared off by the glass flow, and become inclusions in the sheet. Secondary crystals incorporated into the drawn glass are visual defects. Panels with such defects are rejected. Secondary zircon precipitation has been controlled by restricting the weir-root temperature difference to less than about 100° C., thereby limiting the types of glasses that can be fusion formed to the high standards of glass quality required by display manufacturers because only glasses which have the requisite viscosity properties over this temperature range can be used. An isopipe material which did not have the secondary zircon problem would allow the forming process window to be expanded. An expanded operating window, in turn, makes yield improvements possible. Further, some new glasses may require operation in a higher temperature regime where the zircon family is no longer suitable.
Second, zircon also restricts the lifetime and operating temperature range of an isopipe because of its high temperature creep characteristics. Creep is the permanent change in the physical shape of a refractory or other material as a result of an imparted stress usually at elevated temperature. The creep acts in such a way as to relieve the stress, and is usually attributed to grain boundary sliding or material diffusion. An isopipe undergoing creep sags in the middle and deforms the weirs over which the glass flows. When the weirs are no longer straight, the glass flow distribution across the length is disturbed and it becomes more difficult and eventually impossible to manage glass sheet formation, thus ending production.
At high temperature, zircon decomposes to silica liquid and zirconia. Silica liquid at grain boundaries increases the creep rate. This makes firing the refractory a compromise between microstructural quality and creep behavior. Display glass drawn on an isopipe with excessive creep deformation cannot meet the uniform thickness requirements because as discussed above, the weirs deform which changes the mass distribution across the isopipe eventually exceeding the compensational capability of conventional operational tools.
Thus, even though zircon is considered to be a high performance refractory material, in practice, isopipes composed of commercially available zircon exhibit dimensional changes which limit their useful life.
D. Intrinsic Rate of Creep
In view of the foregoing, it is desirable to reduce the intrinsic rate of creep for any material used as an isopipe to: 1) enable use of a wider pipe, 2) extend the fusion draw process to higher temperature glasses (e.g., higher strain point glass that is more compatible with poly-silicon display manufacturing processes), and/or 3) extend the service life of the isopipe and thus minimize process down time and replacement costs.
Analysis shows that the rate of isopipe sag is proportionate to its length raised to the fourth power and inversely proportionate to the square of its height. A doubling in the length of the isopipe (with the same life requirement and temperature capability) requires either a 16 fold decrease in intrinsic creep rate or a four fold increase in height. The current process for fabrication of zircon isopipes (cold isostatic pressing followed by sintering) cannot accommodate a four fold increase in isopipe height. The maximum length for a zircon isopipe which still has a reasonable service life has thus in essence been reached in the art or shortly will be reached with the current isopipe manufacturing technology. Accordingly, the ability to satisfy future requirements of flat panel display manufacturers for larger substrates will be substantially compromised with current technology.
As discussed below, the present disclosure provides isopipes that have significantly improved creep rates compared to isopipes made from commercially available zircon, e.g., well below the 16-fold decrease in creep rate needed to compensate for a doubling in the length of an isopipe. As also discussed below, the isopipes have a silica coating that is compatible with the types of glass compositions used to make substrates for flat panel displays.
Accordingly, these isopipes are well-suited for producing flat glass by the fusion process because they can address some or all of the length, processing temperature, and/or sag problems of isopipes made from existing refractory materials, specifically, commercially available zircon. By addressing these problems, cost reductions can be achieved in, for example, the following ways: (1) longer isopipe lifetimes requiring less rebuild; (2) an expanded process window enabling yield improvements; (3) long term stability of isopipe shape allowing for reduced complexity of operation, especially near the end of an isopipe's life; and/or (4) increased glass delivery temperatures to the isopipe (˜1300° C. and higher) thus allowing the platinum delivery system to be shortened thus reducing material costs.